date and time column as string

In case the format of the datetime column consists only of numbers, the column is interpreted as integer. Leading to problems when combining the date and time column.

A first idea to solve the solution was mentioned in !7 (closed), with the drawback that leading zeros vanished after converting the pandes series from int to string.

The correct type must be given alerady when pd.read_csv() is called.

(This issue was raised by Elizabeth)